The history of the development of the High Electron Mobility Transistor (HEMT) is an outstanding illustration of how a new device can be successfully marketed. In this paper we discuss a key to successful commercialization of new devices.

This paper describes the device fabrication process and characteristics of AlGaN/GaN heterostructure field-effect transistors (HFETs) aimed for millimeter-wave applications. We developed three novel techniques to suppress short-channel effects and thereby enhance high-frequency device characteristics: high-Al-composition and thin AlGaN barrier layers, SiN passivation by catalytic chemical vapor deposition, and sub-100-nm Ti-based gates. The Al0.4Ga0.6N/GaN HFETs with a gate length of 30 nm had a maximum drain current density of 1.6 A/mm and a maximum transconductance of 402 mS/mm. The use of these techniques led to a current-gain cutoff frequency of 181 GHz and a maximum oscillation frequency of 186 GHz.

E-band low-noise amplifier (LNA) monolithic millimeter-wave integrated circuits (MMICs) were developed using pseudomorphic In0.75Ga0.25As/In0.52Al0.48As high electron mobility transistors (HEMTs) with a gate length of 50 nm. The nanogate HEMTs demonstrated a maximum oscillation frequency (fmax) of 550 GHz and a current-gain cutoff frequency (fT) of 450 GHz at room temperature, which is first experimental demonstration that fmax as high as 550 GHz are achievable with the improved one-step-recessed gate procedure. Furthermore, using a three-stage LNA-MMIC with 50-nm-gate InGaAs/InAlAs HEMTs, we achieved a minimum noise figure of 2.3 dB with an associated gain of 20.6 dB at 79 GHz.

We developed a 32-bit pseudorandom number generator (RNG) operating at liquid nitrogen temperature based on HEMT ICs. It generates maximum-length-sequence codes whose primitive polynomial is X47+X42+1 with the period of 247-1 clock cycle. We designed and fabricated three kinds of cryogenic HEMT IC for this system: A 1306-gate controller IC, a 3319-gate pseudorandom number generator (RNG) IC, and a buffer IC containing a 4-kb RAM and 514 gates. We used 0.6-µm gate-length Se-doped GaAlAs/GaAs HEMTs. Interconnects were Al for the first layer and Au/Pt/Ti for the second layer with a SiON insulator between them. The HEMT ICs have direct-coupled FET logic (DCFL) gates internally and emitter-coupled logic (ECL) compatible input-putput buffers. The unloaded basic delay of the DCFL gate was 17 ps/gate with a power consumption of 1.4 mW/gate at liquid nitrogen temperature. We used an automatic cryogenic wafer probe we developed and an IC tester for function tests, and used a high-speed performance measuring system we also developed with a bandwidth of more than 20 GHz for high-speed performance tests. Power dissipations were 3.8 W for the controller IC, 4.5 W for the RNG IC, and 3.0 W for the buffer IC. The RNG IC, the largest of the three HEMT ICs, had a maximum operating clock rate of 1.6 GHz at liquid nitrogen temperature. We submerged a specially developed zirconium ceramic printed circuit board carrying the HEMT ICs in a closed-cycle cooling system. The HEMT ICs were flip-chip-packaged on the board with bumps containing indium as the principal component. We confirmed that the RNG system operates at liquid nitrogen temperature and measured a minimum system clock period of 1.49 ns.

We fabricated 50-nm-gate InAlAs/InGaAs high electron mobility transistors (HEMTs) lattice-matched to InP substrates by using a conventional process under low temperatures, below 300C, to prevent fluorine contamination and suppress possible diffusion of the Si-δ-doped sheet in the electron-supply layer, and measured the DC and RF performance of the transistors. The DC measurement showed that the maximum transconductance gm of a 50-nm-gate HEMT is about 0.91 S/mm. The cutoff frequency fT of our 50-nm-gate HEMT is 362 GHz, which is much higher than the values reported for previous 50-nm-gate lattice-matched HEMTs. The excellent RF performance of our HEMTs results from a shortening of the lateral extended range of charge control by the drain field, and this may have been achieved because the low-temperature fabrication process suppressed degradation of epitaxial structure.